November 1997

Science Briefing

The silver glow of polar moonlight will help illuminate the state of the atmosphere through an Arctic instrument initiative. ACD tested its new high-resolution infrared spectrometer at Thule, Greenland, last month. The device should be sensitive enough to use both sunlight and moonlight in profiling the atmosphere.

Mike Coffey

ACD's Bill Mankin, Mike Coffey, and Jim Hannigan developed the new instrument. Mike and Jim took it to Greenland for a month-long initial deployment in October. It will join several other sensors next spring as part of the Network for the Detection of Stratospheric Change (NDSC), supported by a variety of international scientific agencies.

Because stratospheric ozone and other air chemistry vary so strongly with latitude, the NDSC sites are being organized in sets that straddle various latitudes. Each site and its instruments are expected to operate for 20 years. The Arctic sites include Thule (76 degrees N); Eureka, Canada (80 degrees N); and Spitzbergen, off the coast of Norway (78.5 degrees N). They are joined by other NDSC sites in Hawaii, France, Switzerland, New Zealand, and the Antarctic. "There's quite a lot of international cooperation in this program," says Mike.

The new NCAR spectrometer, unlike its predecessors, can provide information on the vertical distribution of atmospheric constituents, because it can resolve the shape of molecular absorption features. Previous models could only analyze the final amount of light as it reached the earth. With a vertical resolution of 10 kilometers (6 miles), "It's a gross measurement," says Mike, "but we can finally say something about how components are vertically distributed."

Jim Hannigan with ACD's new spectrometer.

Since sunlight is scarce above the Arctic Circle, ACD designed its new spectrometer to use the moon as an additional source of reflected sunlight. Because infrared moonlight is over a million times dimmer than infrared sunlight, "We'll only try it during the full moon," says Mike. A German group reported the first success at lunar spectrometry recently at the Spitzbergen site.

During the October tests, says Mike, "We had some very sunny days for the first few days, and then as soon as we got the instrument ready to go, we had a Class 3 storm, which means everybody's restricted to the area where they're sleeping. Then the sun failed to come out again, so we weren't able to make any actual observations, but we did get a lot of hardware and communications work done."

The spectrometer will be permanently deployed next spring at Thule and operated remotely from ACD, with only a monthly check from technicians in Greenland to change liquid nitrogen and replace the data-storage medium. For now, the instrument is back in Boulder, where ACD will be refining its automated features over the next few months.